1. Field of the Invention
The present invention relates to a GaN related compound semiconductor element used as, for example, a semiconductor amplification element such as a power transistor capable of producing a large current, to a process for producing the same, and to a GaN related compound semiconductor device.
2. Description of the Related Art
A MOS-FET, a HEMT (High Electron Mobility Transistor), or the like, using a GaN related group III-V compound semiconductor, such as GaN or AlGaN, for a channel layer has attracted attention as a device having high breakdown voltage characteristics and being capable of operating at a high temperature with a large electric current. This is because the on-resistance, during the operation, of the device using a GaN related group III-V compound semiconductor is smaller than that of a device using Si, GaAs, or the like, by one digit or more. As an example of such devices, Japanese Patent Application Laid-open Publication No. 2004-260140 describes a group III nitride semiconductor element.
There is a GaN related compound semiconductor element having a ridge portion as shown in FIG. 18, for example. In this GaN related compound semiconductor element, a GaN buffer layer 52, an undoped GaN layer 53, an n+ type GaN drain layer 54, an n− type GaN layer 55, a p type GaN channel layer 56 are stacked on a semi-insulating sapphire substrate 51. An n type GaN source layer 57 having a ridge stripe shape is formed on the p type GaN channel layer 56. In addition, a source electrode 60 is formed over the entire surface of the ridge shape of the n type GaN source layer 57, as well as part of the surface of the p type GaN channel layer 56.
On the other hand, a gate electrode 59 is formed on an insulating film 58 stacked on the surface of the p type GaN channel layer 56. A drain electrode 61 is formed on exposed part of the surface of the n+ type GaN drain layer 54 subjected to mesa etching.
However, in the above-described conventional GaN related compound semiconductor element, the following problem arises. In a process for producing the GaN related compound semiconductor element shown in FIG. 18, the GaN buffer layer 52 to the n type GaN source layer 57 are firstly stacked on the sapphire substrate 51, and thereafter, the n type GaN source layer 57 is shaped into the ridge shape by mesa etching. However, since it is difficult to remove, by wet etching, the GaN related compound semiconductor, which is hard, dry etching using plasma irradiation or the like is generally employed.
When the ridge portion of the n type GaN source layer 57 is formed by this dry etching, it is necessary that the entire n type GaN source layer 57 be removed except the ridge portion by the dry etching. As a result, the exposed surface (the portion indicated by xxx in FIG. 18) of the p type GaN channel layer 56 is often damaged.
If the surface of the p type GaN channel layer 56 is damaged as described above, a Schottky contact is formed in the junction region between the p type GaN channel layer 56 and the source electrode 60. As a result, the contact resistance between the p type GaN channel layer 56 and the source electrode 60 is increased, thus preventing an electric current from flowing in the element. In addition, when the p type GaN channel layer 56 is damaged, the interface state density of the p type GaN channel layer 56 is increased. Accordingly, when a positive voltage is applied to the gate electrode 59, the p type GaN channel layer 56 is not immediately inverted to the n type channel. For this reason, it takes time to form a population inversion state. As a result, since the on-resistance is increased, the element cannot operate at a high speed.
Moreover, in the above-described conventional GaN related compound semiconductor element, when a bias voltage is applied to the gate electrode 59, a depletion layer is formed in the p type GaN channel layer 56 as shown in FIG. 17. However, this depletion layer is unlikely to expand in the lateral directions (the left and right directions in FIG. 17). With no lateral expansion, the depletion layer cannot reach a vicinity of the n type GaN source layer 57. In this case, the population inversion is difficult to occur, so that the on-resistance is increased. As a result, a problem arises that a current does not flow between the drain electrode 61 and the source electrode 60.
Furthermore, in the above-described conventional GaN related compound semiconductor element, the following problem may also occur. FIG. 12 illustrates a wurtzite single crystal structure, and shows the orientations and the like of the structure. The crystal structure of sapphire is represented by a crystal structure of a hexagonal system as shown in FIG. 12. When a GaN related compound semiconductor layer is stacked on a sapphire substrate as shown in FIG. 17 or 18, the c plane (0001) of the sapphire substrate is generally used. The GaN related compound semiconductor layers stacked on the sapphire substrate of the (0001) orientation have a wurtzite crystal structure (the crystal structure shown in FIG. 12) of the (0001) orientation, and also have a crystal polarity allowing the cation element of Ga to grow in the direction of the growth surface (to grow in the c-axis direction). Accordingly, all the GaN related compound semiconductor layers staked on the c plane (0001) of the sapphire substrate grow in the direction of the growth surface on the c plane (0001).
As shown in FIG. 12, the m plane (10-10), which is a prismatic plane of a single crystal, is a fundamental plane constituting each individual single crystal. For this reason, a crystal is likely to be cracked along them plane. Accordingly, when a crack occurs in a wafer, the crack runs along the m planes. This crack sometimes cut off an electrode provided to the element.
As shown in FIG. 17 or FIG. 18, since the gate electrode 59 is formed on the insulating film 58, the gate electrode 59 has a gate capacitance. With an increased gate capacitance, it takes a longer time to switch ON and OFF of the element even when an ON-OFF switching voltage is applied to the gate electrode 59. In addition, since an increase in the gate capacitance causes also an increase in the power consumption, the area of the gate electrode 59 is generally made as small as possible. For this purpose, the gate electrode 59 is provided in the stripe shape with a small wiring width. Accordingly, such gate electrode 59 is weak against a breaking force in a direction substantially perpendicular to the extending direction of the stripe shape.
Since the p type GaN channel layer 56 grows in the direction of the growth surface on the c plane as described above, the directions of the m planes are sometimes aligned in a direction substantially perpendicular to the direction in which the gate electrode 59 extends in the stripe shape. In such a case, when a crack occurs in the p type GaN channel layer 56, the crack runs along the m planes. Accordingly, the gate electrode 59 is easily cut off in a direction of the shorter dimension of the gate electrode 59. When the gate electrode 59 is cut off, the population inversion does not occur in the p type GaN channel layer 56, and thus the element is not switched to the ON state. As a result, a current does not flow in the element.
The present invention has been made for the purpose of solving the above-described problems. An object of the present invention is to provide a GaN related compound semiconductor element which is capable of reducing the on-resistance by securely causing a population inversion to occur in a channel layer, and to provide also a process for producing the GaN related compound semiconductor element, as well as a GaN related compound semiconductor device.